FIG. 8 is a diagram showing a structure of a common solid-state laser device according to a related art. A direct current flows to a laser diode 1 to emit light, a laser medium 2 is excited, and resonance occurs between a total reflection mirror 3 and a partially reflection mirror 4, so that laser light is obtained. The thus obtained laser light is expanded in parallel by using an enlargement lens 5 and a parallel lens 6, and is condensed at an end face of an optical fiber 8 by using an optical fiber incident lens 7. The condensed laser light passes through the optical fiber 8 and is guided to a predetermined location through a working head 9.
FIG. 9 is a schematic diagram showing a structure of a portion where the solid laser medium (hereinafter referred to as a laser medium, as needed) 2 is excited by the laser diode 1. A plurality of laser diodes 1 (1a, 1b, 1c and 1d) are connected in series to a direct current power source 10, and apply excitation light to the cylindrical laser medium 2 via a light guide 11. The excitation light may be applied directly to the laser medium 2 without using the light guide 11. Since a current is used to control the output of the laser diode 1, a direct current power source for controlling the current is frequently employed as the direct current power source 10.
The output level of one laser diode 1 is several tens of W; however, since the output required for the entire solid-state laser device is several hundred W to several kW, it is not rare for several tens of laser diodes 1 to be connected in series.
In this example, the excitation light supplied by the laser diode 1 is applied from one direction. However, as is shown in FIG. 10, the laser diodes 1 (1a, 1b, 1c and 1d) may be arranged at 90 degree intervals, and the laser medium 2 is irradiated in four directions by the excitation light.
While the laser diode 1 is in use, due to occurrence of an abnormality, such as a short or an open circuit fault, no current may flow and no excitation light may be emitted from the laser diode 1. In order to detect an abnormal state (hereinafter referred to as a “fault”, as needed) wherein no excitation light is emitted from the laser diode 1, the following method is conventionally employed.
One of the laser diodes 1 used for the solid-state laser device generates excitation light to provide heat having several tens of W. For the detection of a fault at a laser diode 1 in the mW order for image formation, as is shown in FIG. 11 the excitation light emitted from the laser diode 1 is measured using a photodiode 16, and the fault is detected by a light intensity measuring unit 17 and a fault detector 18.
In this arrangement, the photodiode 16 must be provided in addition to the laser diode 1. Since it is necessary that the exciting of the laser medium 2 is performed as efficiently as possible, the laser diode 1 is arranged to be near the laser medium 2 and the light guide 11. Thus, it is very difficult to ensure a location where the photodiode 16 is arranged, and design thereof is also difficult. In addition, it is necessary that a part of the excitation light generated by the laser diode 1 is fetched in the photodiode 16, and thus, since all the excitation light emitted from the laser diode 1 can not be applied to the laser medium 2, efficiency is deteriorated.
As is described above, in the arrangement of the photodiode 16, etc., there is a problem that the device becomes complicated and the irradiation efficiency is reduced. Therefore, in the laser diode 1 used for a solid-state laser device, generally, the current input to the laser diode 1 is employed to determine whether excitation light emitted from the laser diode 1 is irradiated. Specifically, a status of the current flowing to the laser diode 1 is fed back to a controller (not shown). When there is no current flow, it is determined that an open fault occurs at the laser diode 1, or when there is an overcurrent flow, it is determined that a short circuit occurs at the laser diode 1, so that no emission of excitation light by the laser diode 1 is detected.
FIG. 12 is a voltage-current characteristic graph for a laser diode. When, for the above described current feedback method, a commonly used power source of a current control type is employed as the power source, even if a short circuit occurs at the laser diode, the resistance does not become 0 Ω due to line resistance and a constant current is maintained (set to a state at point E in FIG. 12). Further, since there is no overcurrent flow, the detection of a fault cannot be performed. Furthermore, when an open fault has occurred, the flow of the current is halted and the fault can be detected. However, generally, since several tens of laser diodes are connected in series to provide a large output, if an open fault has occurred at one of them, it is difficult to pinpoint which one of the laser diodes is opened.
As is described above, so long as a commonly used power source of a current control type is employed, the solid-state laser device according to a related art can not detect that emission of excitation light has been halted due to a short-circuit at a laser diode. Further, when a plurality of laser diodes are employed, it is difficult to pinpoint which one of the laser diodes is in fault.
To resolve the above problems, it is one objective of the present invention to provide a solid-state laser device that can detect, without using a photodiode, a state where a laser diode is not emitting excitation light, and at the same time, can easily pinpoint which one of the laser diodes is in fault.